It is commonly understood that when a first circuit is coupled to a second circuit (or an external circuit) so that signals may be transmitted from the external circuit to the first circuit, a proper coupling circuit is needed to connect the external circuit to the first circuit because the external circuit may have a different DC bias than that of the first circuit. A capacitive coupling circuit is commonly used in signal paths to isolate the first circuit from the external circuit so that the DC bias of the external circuit does not directly affect the operation of the first circuit.
FIG. 1 illustrates a capacitive coupling between two circuits with different DC biases. Referring to FIG. 1, a first circuit 20 and an external circuit 12 are coupled together in signal paths via capacitors 18A, 18B that form a capacitive coupling. The external circuit 12 is supplied with a DC bias Vext 16 that provides a DC operating point for the external circuit 12. Further, the external circuit 12 includes outputs 14A, 14B that output a signal to capacitors 18A, 18B. Circuit 20 may include a differential amplifier 30 that is coupled to capacitors 18A, 18B for receiving the signal from the external circuit 12. The amplifier 30 also includes differential outputs 26A, 26B that provide the signal to other portions of circuit 20. Additionally, the amplifier 30 includes a common mode input 22 that receives a DC bias Vcom 28 based on which the amplifier 30 operates. Commonly, the DC bias Vcom 28 on circuit 20 is not leveled with the DC bia Vext 16 for the external circuit 12. Therefore, capacitive coupling circuits such as capacitors 18A, 18B may be used to isolate the external circuit 12 from circuit 20 and to ensure that circuit 20 operates properly without the DC interference from Vext 16.
While a capacitive coupling may help isolate circuit 20 from the DC bias of external circuit 12, the capacitive coupling degraded the low frequency performance of the circuit system. Capacitors 18A, 18B, along with the finite input impedance network in amplifier 30, act as a high-pass filter that removes low frequency components contained in the signal received from the external circuit 12, and thus degrade the low frequency performance for the whole system 10.
To reduce the side effects of degrading low frequency performance, the capacitive coupling is commonly designed to include high-capacitance capacitors. High capacitance means that the physical sizes of capacitors 18A, 18B need to be very large. However, large capacitors may not be realistic in certain applications such as cell phones.